The invention relates to a method for extending the dynamic range in measurements carried out with a directional coupler, such as antenna measurements. In the method, a signal is supplied to the object under analysis, such as an antenna, through a first port of the measuring directional coupler, part of the signal leaking as a leak signal through a second port of the measuring directional coupler directly towards measuring means, and the signal returning from the object under analysis is measured by the measuring means. The invention also relates to a device for carrying out the method. The device comprises (i) transmitter means for generating an electric measurement signal, (ii) a measuring directional coupler, to which the transmitter means are coupled for coupling the measurement signal to the object to be measured, and (iii) measuring means for measuring the signal returning from the object under analysis. In particular, the invention can be applied to measuring the standing-wave ratio (SWR) of an antenna in order to observe the condition of the antenna.
In measurements such as those which are mentioned above the situation is the following: the signal reflected back from the object to be measured, such as an antenna, is, when summing to the signal leaking from the directional coupler straight in the direction of the measuring means, at a random phase, e.g. depending on variations in the electric length of the antenna cable. The magnitude of the error caused by summation is thus dependent on the phase difference between the returning signal and the leaking signal, which varies, e.g., as a function of the electric length of the antenna cable. Known measuring solutions aim at the best possible directivity of the directional coupler supplying the measurement signal to the antenna, in order to render as small as possible the spurious signal leaking from the directional coupler straight in the direction of the measuring means and summing to the actual signal to be measured. The effect of other spurious signals such as those reflected from the coupling points has been diminished by calibration and by means of modification tables and correction factors. The obtained directivity of the directional coupler is, however, rarely more than 30 dB; in general the directivity is in the range of 20 . . . 30 dB, as the directional coupler is to be realised on the printed circuit board as a micro strip structure, which is production-technically advantageous, due to easy reproducibility. Owing to the limited directivity, the measuring of low signal levels, such as the signal reflected back from an antenna demonstrating a minor reflection loss, becomes difficult when the (useful) signal to be measured sums to a spurious signal leaking from the directional coupler. On the other hand, the useful signal to be measured sums also to other spurious signals passing to the measuring point, such as the signals reflected from the coupling points. In some cases, these other spurious signals may be greater in magnitude than the above-mentioned leaking signal, thus preventing the measuring of low signal levels.